This invention relates generally to methods for investigating subsurface earth formations, and more specifically to methods for estimating the shear wave travel time in earth formations adjacent to a borehole.
It is known in acoustic well logging to utilize one or more acoustic pulse transmitters spaced axially along a borehole from one or more acoustic receivers as illustrated in U.S. Pat. Nos. 4,131,875 and 4,346,460. When an acoustic pulse is generated by a transmitter a complex acoustic wave is produced. This acoustic wave travels both radially away from the borehole into the formation and axially towards the receiver. The receivers convert the received acoustic wave into an electrical wave form for processing.
Acoustic waves are broadly classified into two groups: longitudinal waves and transverse waves. Each of these groups comprises several wave types. Most common in the field of acoustic well logging are the compressional waves and the shear waves. Compressional waves are propogated in the compressional mode, that is, the direction of propagation is parallel to the direction of particle displacement. The compressional wave propagates in the borehole fluid, is critically refracted into the surrounding formation in a compressional fashion and returns through the borehole fluids to the instrument receiver as a compressional wave. Gases, liquids and solids tend to oppose compression; therefore, compressional waves can be propagated through them.
Shear waves are propagated in the shear mode, that is, the direction of propagation is perpendicular to the direction of particle displacement. The shear wave propagates as a compressional wave in the borehole fluid, is critically refracted into the formation as a shear wave and returns to the receivers as a compressional wave through the borehole fluid. Because of their rigidity, solids tend to oppose shearing. Therefore, shear waves can be propagated through solids. On the other hand, liquids and gases have no rigidity and cannot oppose shearings, shear waves cannot be propagated through them.
Each acoustic pulse of sufficient amplitude produce an acoustic signal at each receiver. Shortly after pulse initiation, the acoustic events arrive at the receiver in the following order. First, the compressional wave; next, the shear wave; and finally a group of later waves, such as the mud wave and the boundary wave. Each wave arrives at a receiver as a wave train that comprises a large number of cycles. Because of the inertia of particles, the amplitude of successive cycles increases at first, reaches a maximum, and then decreases. The time of the first arrival, the amplitude of the cycles, and the persistence of the wave train are different for each formation wave. They are also dependent upon the type of formation and are therefore characteristic of the formation.
In common acoustic logging practice the shear wave arrives before the end of the compressional wave. This overlap produces wave interference and, therefore, distortion of some cycles of the signal. Also, the compressional wave always exists, the shear wave can typically be determined only when the shear velocity in the formation is greater than the compressional velocity in the mud. This is generally only the case in consolidated formations.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing methods for estimating in unconsolidated, shaly formations and/or gas bearing formations the shear wave travel time and other, logging derived information.